JP6490603B2 - Predistortion circuit and communication device - Google Patents

Description

  Embodiments described herein relate generally to a predistortion circuit and a communication device.

  An amplifier used in a communication apparatus is required to have linear characteristics from the viewpoint of improving signal quality and reducing interference power to other frequency channels. On the other hand, from the viewpoint of reducing the power consumption of the communication device, a class AB amplifier having a lower linear characteristic than class A may be used instead of class A operation with high linearity. In addition, a Doherty amplifier may be used to increase power efficiency. When a class AB or Doherty type amplifier is used, further improvement in linearity is required as compared with the case where a class A amplifier is used.

  As a method for improving the linear characteristic of the amplifier, there is a method using a digital predistortion circuit that improves distortion generated in the amplifier by digital signal processing. However, a relay station that uses a radio wave in the GHz band uses a method (a non-reproducing relay method) that amplifies and transmits a received analog signal without reproducing a signal to be transmitted / received into a digital signal. There is. In such a relay station, in order to apply the digital predistortion circuit, it is necessary to add a module that converts the signal into a digital signal, performs digital signal processing, and converts the signal into an analog signal. However, in a relay station using a non-regenerative relay system, application of a digital predistortion circuit may not be preferred from the viewpoint of cost or the like, and predistortion is performed on an analog signal.

  A predistortion circuit for an analog signal is usually configured using electronic components such as a diode and an FET. The nonlinear distortion of the diode or FET is approximately generated in the preceding stage so as to cancel the distortion generated in the subsequent amplifier. In such predistortion circuit design, circuit configurations such as diodes and FETs are often experimentally determined by trial and error so that the operation according to the characteristics of the amplifier provided in the subsequent stage is performed, and predistortion operates. Since the dynamic range to be used is narrow, it may take time to study and design.

JP 2001-320245 A

Steve C. Cripps, "Advanced Techniques in RF Power Amplifier Design", 1st Edition, Artech House Print on Demand, June 15, 2002

  The problem to be solved by the present invention is to provide a predistortion circuit and a communication device that can reduce the time required for design or examination.

  The predistortion circuit according to the embodiment includes a first multiplication unit, first and second level adjustment units, and a vector modulation unit. The first multiplication unit calculates a first square signal obtained by squaring the input analog signal. The first level adjustment unit acquires the first adjustment signal from the first square signal by performing level adjustment. The second level adjustment unit acquires the second adjustment signal from the first square signal by performing level adjustment. The vector modulation unit generates a first analog signal and a second analog signal having a phase difference of 90 degrees from the analog signal, a first multiplication result of the first adjustment signal and the first analog signal, The second adjustment signal and the second multiplication result of the second analog signal are combined and an output signal obtained by the combination is output.

FIG. 3 is a block diagram illustrating a configuration example of a predistortion circuit according to the first embodiment. The figure which shows operation | movement of a predistortion circuit. The graph which shows the improvement of the characteristic by a predistortion circuit. The graph which shows improvement of IM3 by the presence or absence of a predistortion circuit. The block diagram which shows the structural example of the predistortion circuit in 2nd Embodiment. The block diagram which shows the structural example of the predistortion circuit 3 in 3rd Embodiment. The block diagram which shows the structural example of the relay apparatus in 4th Embodiment. The block diagram which shows the structural example of the transmission system 60 in 5th Embodiment.

  Hereinafter, a predistortion circuit and a communication device according to embodiments will be described with reference to the drawings. Note that, in the following embodiments, components with the same reference numerals perform the same operations, and redundant descriptions are omitted as appropriate.

(First embodiment)
FIG. 1 is a block diagram illustrating a configuration example of the predistortion circuit 1 according to the first embodiment. The predistortion circuit 1 applies distortion to an analog signal based on distortion characteristics obtained by approximating distortion characteristics of an amplifier provided in a subsequent stage with a third-order polynomial. The predistortion circuit 1 improves the linear characteristic of the amplifier by giving in advance an analog signal with distortion that suppresses distortion generated in the amplifier.

When the input and output of the predistortion circuit 1 are vin and Vpd, and the coefficient of the third-order nonlinear term of the predistortion circuit 1 is b3, the input / output characteristics of the predistortion circuit 1 are expressed by equation (1). . It should be noted that ^ (hat) used in the following formulas indicates a power, for example, vin ^ 3 indicates the third power of vin.
Vpd = vin + b3 × vin ^ 3 (1)

Here, the input / output characteristics of the amplifier provided in the subsequent stage of the predistortion circuit 1 are approximated as shown in Expression (2). Vout is the output of the amplifier. Note that a3 is a coefficient of the third-order nonlinear term of the amplifier.
Vout = Vpd + a3 × Vpd ^ 3 (2)

When Expression (1) is substituted into Expression (2), the input / output characteristics of the circuit including the predistortion circuit 1 and the amplifier are expressed by Expression (3).
Vout = (vin + b3 × vin ^ 3) + a3 × (vin + b3 × vin ^ 3) ^ 3
= vin + (b3 + a3) vin ^ 3 + 3 × a3 × b3 × vin ^ 5
+ 3 × a3 × b3 ^ 2 × vin ^ 7 + a3 × b3 ^ 3 × vin ^ 9 (3)

Here, by mounting the predistortion circuit 1 having the characteristic satisfying the expression (4), the distortion of the amplifier expressed by the third-order term can be canceled.
b3 = -a3 (4)

  The predistortion circuit 1 includes an input terminal IN, attenuators 11 and 21, multiplier circuits 12 and 22, filters 13 and 23, level adjustment units 14, 15, 24 and 25, a delay element 31, and vector modulation. Device 32 and an output terminal OUT. The vector modulator 32 includes a 90 degree hybrid, mixers 34 and 34, and a synthesizer 36.

  An analog signal input from the input terminal IN is input to the attenuators 11 and 21 and the delay element 31. The attenuator 11 attenuates the input analog signal and outputs it. The signal output from the attenuator 11 is branched into two and each is input to the multiplier circuit 12. The multiplier circuit 12 multiplies two input signals. The filter 13 extracts an envelope component from the components included in the signal output from the multiplication circuit 12 and outputs the extracted envelope component to the level adjustment unit 14. The filter 13 removes unnecessary components and noise generated when the analog signals (fundamental waves) in the multiplication circuit 12 are multiplied. As the filter 13, for example, a low-pass filter determined according to the frequency of the analog signal is used.

  The level adjustment unit 14 performs level adjustment on the signal output from the filter 13. The level adjustment in the level adjustment unit 14 is adjustment of the amplitude of the signal, and an output is obtained by expanding or reducing (amplifying or attenuating) the envelope component of the input signal. The level adjustment unit 15 performs level adjustment (step-down or step-up) with respect to a constant voltage supplied from the outside. The signal output from the level adjustment unit 14 and the constant voltage output from the level adjustment unit 15 are combined. The first adjustment signal obtained by the synthesis is input to the vector modulator 32. The level adjustment in the level adjustment units 14 and 15 is determined according to the characteristics of the amplifier provided in the subsequent stage of the predistortion circuit 1. For example, when the level adjustment in the level adjustment units 14 and 15 is only attenuation, a variable resistor or the like can be used as the level adjustment units 14 and 15.

  The attenuator 21, the multiplication circuit 22, the filter 23, the level adjustment unit 24, and the level adjustment unit 25 perform the same operations as the attenuator 11, the multiplication circuit 12, the filter 13, the level adjustment unit 14, and the level adjustment unit 15, respectively. The signal output from the level adjustment unit 24 and the constant voltage output from the level adjustment unit 25 are combined. The second adjustment signal obtained by the synthesis is input to the vector modulator 32. Since the configuration from the attenuator 11 to the level adjustment unit 14 and the configuration from the attenuator 21 to the level adjustment unit 24 are symmetric, the signal delay in each path is the same.

  The delay element 31 delays an input analog signal and outputs it. The delay time in the delay element 31 is the time required for the analog signal input to the input terminal IN to reach the mixer 34 via the delay element 31 and the 90-degree hybrid 33 and the analog signal input to the input terminal IN. The time required for the signal to reach the mixer 34 via the attenuator 11, the multiplier circuit 12, the filter 13, and the level adjustment unit 14 is equal.

  The analog signal to which the delay is given by the delay element 31 is input to the vector modulator 32. The analog signal input to the vector modulator 32 is input to the 90-degree hybrid 33. The 90-degree hybrid 33 branches an input analog signal into two first analog signals and second analog signals having a phase difference of 90 degrees. The 90-degree hybrid 33 outputs the first analog signal to the mixer 34 and outputs the second analog signal to the mixer 35.

  The mixer 34 multiplies the first analog signal input from the 90-degree hybrid 33 and the first adjustment signal, and outputs the first multiplication result obtained by the multiplication to the synthesizer 36. The first multiplication result output from the mixer 34 includes an analog signal (fundamental wave) and a signal including a third-order nonlinear component corresponding to the first adjustment signal generated by the level adjustment units 14 and 15. It is. The mixer 35 multiplies the second analog signal input from the 90-degree hybrid 33 and the second adjustment signal, and outputs the second multiplication result obtained by the multiplication to the synthesizer 36. The second multiplication result output from the mixer 35 includes an analog signal (fundamental wave) and a signal multiplied by 3 according to the second adjustment signal generated by the level adjustment units 24 and 25. The combiner 36 combines the first multiplication result and the second multiplication result. The synthesizer 36 outputs a signal obtained by the synthesis as an output signal from the output terminal OUT to the outside.

  By adjusting the level b in the level adjustment units 14, 15, 24, and 25, the coefficient b3 of the third-order nonlinear term of the predistortion circuit 1 is set to a value satisfying the equation (4), distortion caused by the third-order term is reduced. Can be suppressed.

  By using the vector modulator 32, the predistortion circuit 1 can also suppress AM-PM distortion in addition to suppression of AM-AM distortion in the amplifier at the subsequent stage. This point will be described with reference to FIG. FIG. 2 is a diagram illustrating the operation of the predistortion circuit 1. When the level of the input analog signal is sufficiently low, the level of the second-order nonlinear signal output from the multiplier circuits 12 and 22 is negligibly low, and the first and second adjustment signals are the level adjustment unit 15, It is determined by the constant voltage output from 25. At this time, the amplitude and phase of the output signal output from the vector modulator 32 are controlled by the constant voltage of the level adjusters 15 and 25. In FIG. 2, the signal of the fundamental wave of the analog signal determined by the level adjustment units 15 and 25 is indicated by a vector A.

  When the level of the input analog signal increases, the level of the second-order nonlinear signal output from the multiplier circuits 12 and 22 becomes sufficiently high, and the signal for suppressing the third-order distortion of the subsequent amplifier is vector-modulated. Occurs at the output of the device 32. In FIG. 2, a signal for suppressing the third-order distortion is indicated by a vector B. When the level of the input analog signal is high, the output of the vector modulator 32 is represented by a vector C obtained by combining the vector A and the vector B. In the example shown in FIG. 2A, the output of the vector modulator 32 when the level of the analog signal is high is expanded in amplitude and the phase is counterclockwise compared to the vector A when the level of the analog signal is low. It is rotating.

  When the level of the input analog signal is high and the phase of the output signal output from the vector modulator 32 is to be rotated clockwise, as shown in FIG. The level adjusters 14 and 24 are controlled so that the angle B is smaller than the angle of the vector A with respect to the I axis. In FIG. 2B, when the vector A and the vector C are compared, the amplitude of the output signal is expanded when the level of the analog signal is increased, as in FIG. Is clockwise as shown in FIG. 2 (A).

  In this way, by giving the predistortion circuit 1 an analog signal with the characteristic of the amplifier provided in the subsequent stage of the predistortion circuit 1, particularly the characteristic relating to the third-order term (a signal multiplied by 3), the output of the amplifier is obtained. The linear characteristic at can be improved.

  FIG. 3 is a graph showing the improvement of characteristics by the predistortion circuit 1. In the graph shown in FIG. 3, the vertical axis indicates the gain, and the horizontal axis indicates the level of the input analog signal. Here, the input analog signal vin = 1 is 0 dB. Further, by setting a3 = -0.109, when the predistortion circuit 1 is not used (no PD), the input level 0 dB corresponds to the 1 dB compression point. As shown in FIG. 3, when the predistortion circuit 1 is used (with PD), the gain compression is delayed, and the linear characteristic is improved as compared with the case where the predistortion circuit 1 is not used (without PD). I understand.

  Next, characteristics when a two-wave signal including two-wave components is input to the amplifier will be described. In this case, the distortion component generated at the output of the amplifier includes components called IM3, IM5, and the like as the frequency of the signal (IM1) increases. The component of IM3 is generated not only by the third power of vin in the equation (3) but also by the fifth, seventh, ninth,..., But by canceling the third power, the linear characteristic is reduced. It can be greatly improved.

Hereinafter, an operation when a two-wave signal is input to the predistortion circuit 1 will be described. The input vin of the predistortion circuit 1 is expressed by equation (5). In Equation (5), ω is an RF angular frequency, and 2 × Δω is a detuning frequency of two waves.
vin = V × (cos ((ω-Δω) t) + cos ((ω + Δω) t)
= 2 × V × cos (Δωt) × cos (ωt)
= V (Δωt) × cos (ωt) (5)

The input / output characteristics of the amplifier when the predistortion circuit 1 is not used are expressed by Expression (6).
Vout = V (Δωt) × cos (ωt) + a3 × V (Δωt) ^ 3 × cos (ωt) ^ 3
= V (Δωt) × cos (ωt) + a3 × 1/4 × V (Δωt) ^ 3 × (3 × cos (ωt) + cos (3ωt))
= (V (Δωt) + a3 × 3/4 × V (Δωt) ^ 3) × cos (ωt)
+ A3 × 1/4 × V (Δωt) ^ 3 × cos (3ωt) (6)

In Equation (6), the term (cos (ωt)) represents the fundamental wave component, and the term (cos (3ωt)) represents the third harmonic component, where V (Δωt) = 2V ( cos (Δωt)), the fundamental wave component is divided into an IM1 component and an IM3 component as shown in equation (7).
Vout (fundamental wave)
= (2 × V × cos (Δωt) + a3 × 3/4 × 8 × V ^ 3 × cos (Δωt) ^ 3) × cos (ωt)
= (2 × V × cos (Δωt) + a3 × 6 × V ^ 3 × (3/4 × cos (Δωt) + 1/4 × a3 × cos (3Δωt)) × cos (ωt)
= (2 × V + a3 × 9/2 × V ^ 3) × cos (Δωt) × cos (ωt)
+ (A3 × 3/2 × V ^ 3) × cos (3Δωt) × cos (ωt) (7)

In Expression (7), a term including (cos (Δωt)) is an IM1 component, and a term including (cos (3Δωt)) is an IM3 component. From the equation (7), the following equation group (8) is obtained.
IM1 (low) = (V + 9/4 × a3 × V ^ 3) × cos (ωt− Δωt)
IM1 (high) = (V + 9/4 × a3 × V ^ 3) × cos (ωt + Δωt)
IM3 (low) = (3/4 × a3 × V ^ 3) × cos (ωt−3Δωt) (8)
IM3 (high) = (3/4 × a3 × V ^ 3) × cos (ωt + 3Δωt)

The input / output characteristics when the predistortion circuit 1 is provided can be expressed as follows.
Vout = V (Δωt) (cos (ωt))
+ (B3 + a3) × V (Δωt) ^ 3 × cos (ωt) ^ 3
+ 3 × a3 × b3 × V (Δωt) ^ 5 × cos (ωt) ^ 5
+ 3 × a3 × b3 ^ 2 × V (Δωt) ^ 7 × cos (ωt) ^ 7
+ A3 × b3 ^ 3 × V (Δωt) ^ 9 × cos (ωt) ^ 9
= V (Δωt) × cos (ωt)
+ (B3 + a3) × V (Δωt) ^ 3 × 1/4 × (3 × cos (ωt) + cos (3ωt))
+ 3 × a3 × b3 × V (Δωt) ^ 5 × 1/16 × (10 × cos (ωt) + 5 × cos (3ωt) + cos (5ωt))
+ 3 × a3 × b3 ^ 2 × V (Δωt) ^ 7 × 1/64 × (35 × cos (ωt) + 21 × cos (3ωt) + 7 × cos (5ωt) + cos (7ωt))
+ A3 × b3 ^ 3 × V (Δωt) ^ 9 × 1/256 × (126 × cos (ωt) + 84 × cos (3ωt) + 36 × cos (5ωt) + 9 × cos (7ωt)
+ Cos (9ωt)) (9)

The fundamental wave component is expressed by Expression (10).
Vout (fundamental wave) = (V (Δωt)
+ (B3 + a3) × V (Δωt) ^ 3 × 1/4 × 3
+ 3 × a3 × b3 × V (Δωt) ^ 5 × 1/16 × 10
+ 3 × a3 × b3 ^ 2 × V (Δωt) ^ 7 × 1/64 × 35
+ A3 × b3 ^ 3 × V (Δωt) ^ 9 × 1/256 × 126) × cos (ωt) (10)

Similarly, from V (Δωt) = 2 × V × cos (Δωt), the fundamental wave component is divided into an IM1 component and an IM3 component as follows.
Vout (fundamental wave) = (2 x V x cos (Δωt)
+ 8 × (b3 + a3) × V ^ 3 × 1/4 × 3 × 1/4 × (3 × cos (Δωt) + cos (Δ3ωt))
+ 32 × 3 × a3 × b3 × V ^ 5 × 1/16 × 10 × 1/16 × (10 × cos (Δωt) + 5 × cos (Δ3ωt) + cos (Δ5ωt))
+ 128 × 3 × a3 × b3 ^ 2 × V ^ 7 × 1/64 × 35 × 1/64 × (35 × cos (Δωt) + 21 × cos (Δ3ωt) + 7 × cos (Δ5ωt) + cos (Δ7ωt))
+ 512 × a3 × b3 ^ 3 × V ^ 9 × 1/256 × 126) × 1/256 × (126 × cos (Δωt) + 84 × cos (Δ3ωt) + 36 × cos (Δ5ωt) + 9cos (Δ7ωt) + cos (Δ9ωt) ) × cos (ωt)
(11)

Here, the IM1 component and the IM3 component are expressed by Expression (12) and Expression (13).
Vout (fundamental wave) (IM1) = [2 x V
+ 8 × (b3 + a3) × V ^ 3 × 1/4 × 3 × 1/4 × 3
+ 32 × 3 × a3 × b3 × V ^ 5 × 1/16 × 10 × 1/16 × 10
+ 128 × 3 × a3 × b3 ^ 2 × V ^ 7 × 1/64 × 35 × 1/64 × 35
+ 512 × a3 × b3 ^ 3 × V ^ 9 × 1/256 × 126 × 1/256 × 126] × cos (ωt) × cos (Δωt)
= [2 × V
+ 9/2 × (b3 + a3) × V ^ 3
+ 75/2 × b3 × V ^ 5
+ 3675/32 × a3 × b3 ^ 2 × V ^ 7
+ 3969/32 × a3 × b3 ^ 3 × V ^ 9] × cos (ωt) × cos (Δωt) (12)

Vout (fundamental wave) (IM3) = [8 x (b3 + a3) x V ^ 3 x 1/4 x 3 x 1/4
+ 32 × 3 × a3 × b3 × V ^ 5 × 1/16 × 10 × 1/16 × 5
+ 128 × 3 × a3 × b3 ^ 2 × V ^ 7 × 1/64 × 35 × 1/64 × 21
+ 512 × a3 × b3 ^ 3 × V ^ 9 × 1/256 × 126 × 1/256 × 84] × cos (ωt) × cos (Δ3ωt)
= [3/2 × (b3 + a3) × V ^ 3
+ 75/4 × a3 × b3 × V ^ 5
+ 2205/32 × a3 × b3 ^ 2 × V ^ 7
+ 1323/16 × a3 × b3 ^ 3 × V ^ 9] × cos (ωt) × cos (Δ3ωt) (13)

Thus, the formula group (14) is obtained.
IM1 (low) = [V + 9/4 × (b3 + a3) × V ^ 3 + 75/4 × b3 × V ^ 5 + 3675/64 × a3 × b3 ^ 2 × V ^ 7
+ 3969/64 × a3 × b3 ^ 3 × V ^ 9] × cos (ωt−Δωt)
IM1 (high) = [V + 9/4 × (b3 + a3) × V ^ 3 + 75/4 × b3 × V ^ 5 + 3675/64 × a3 × b3 ^ 2 × V ^ 7
+ 3969/64 × a3 × b3 ^ 3 × V ^ 9] × cos (ωt + Δωt)
IM3 (low) = [3/4 × (b3 + a3) × V ^ 3 + 75/8 × a3 × b3 × V ^ 5 + 2205/64 × a3 × b3 ^ 2 × V ^ 7
+ 1323/32 × a3 × b3 ^ 3 × V ^ 9] × cos (ωt−3Δωt)
IM3 (high) = [3/4 × (b3 + a3) × V ^ 3 + 75/8 × a3 × b3 × V ^ 5 + 2205/64 × a3 × b3 ^ 2 × V ^ 7
+ 1323/32 × a3 × b3 ^ 3 × V ^ 9] × cos (ωt + 3Δωt)
(14)

  The graph which plotted the difference between IM1 and IM3 in the formula group (8) when the predistortion circuit 1 is not used and the formula group (14) when the predistortion circuit 1 is used as IM3 (dBc) is shown. FIG. 4 is a graph showing IM3 improvement with and without the predistortion circuit 1. In the graph shown in FIG. 4, the vertical axis represents IM3 (dBc), and the horizontal axis represents the level of the input analog signal. In the graph shown in FIG. 4, V = 1, a3 = -0.109, and b3 = 0.109. As shown in the graph of FIG. 4, when the predistortion circuit 1 is used, it can be seen that IM3 is improved by about 10 dB at the input level of −10 dB.

  According to the predistortion circuit 1 of the first embodiment, the level adjustment units 14 and 24 that adjust the coefficient for the third-order term and the level adjustment units 15 and 25 that adjust the coefficient for the first-order term (fundamental wave). And can be controlled independently. Thereby, distortion suppression based on the third-order term contributing to improvement of the linear characteristic of the amplifier can be performed independently, and the time required for design or examination can be reduced. Further, by using the vector modulator 32, it is possible to compensate for a phase change that occurs when passing through the amplifier, and to improve the AM-PM characteristic.

  Further, since the attenuators 11 and 21 are provided, it is easy to control a component that suppresses a third-order term appearing in the first and second adjustment signals. As a result, it is possible to select the level of the analog signal at the start of distortion suppression based on the third-order term that contributes to improvement of the linear characteristics of the amplifier, and to reduce the time required for design or examination. . In addition, when handling a signal in the intermediate frequency band, each block provided in the predistortion circuit 1 can be configured by combining commercially available electronic components, and cost can be reduced.

(Second Embodiment)
FIG. 5 is a block diagram illustrating a configuration example of the predistortion circuit 2 according to the second embodiment. Similar to the predistortion circuit 1 of the first embodiment, the predistortion circuit 2 applies distortion to an analog signal based on distortion characteristics obtained by approximating distortion characteristics of an amplifier provided in a subsequent stage with a third-order polynomial. The predistortion circuit 2 improves the linear characteristic of the amplifier by preliminarily giving the analog signal distortion that cancels distortion generated in the amplifier.

  The predistortion circuit 2 includes an input terminal IN, an attenuator 11, a multiplier circuit 12, a filter 13, level adjusters 14, 15, 24, 25, a delay element 31, a vector modulator 32, and an output terminal. OUT. The predistortion circuit 2 has a configuration in which a doubled signal input to each of the level adjusting units 14 and 24 is generated by the attenuator 11, the multiplier circuit 12, and the filter 13, and the predistortion circuit of the first embodiment. Different from 1. The circuit scale can be reduced by sharing the configuration for generating the doubled signal.

(Third embodiment)
FIG. 6 is a block diagram illustrating a configuration example of the predistortion circuit 3 in the third embodiment. The predistortion circuit 3 applies distortion to an analog signal based on distortion characteristics obtained by approximating distortion characteristics of an amplifier provided in a subsequent stage with a fifth-order polynomial. The predistortion circuit 3 improves the linear characteristic of the amplifier by preliminarily giving the analog signal distortion that cancels distortion generated in the amplifier.

  The predistortion circuit 3 includes an input terminal IN, an attenuator 11, multiplication circuits 12 and 16, filters 13 and 17, level adjustment units 14, 15, 18, 24, 25, and 28, a delay element 31, A vector modulator 32 and an output terminal OUT are provided. The predistortion circuit 3 is different from the predistortion circuit 2 of the second embodiment in that the predistortion circuit 3 includes a multiplication circuit 16, a filter 17, and level adjustment units 18 and 28.

  The multiplication circuit 16 branches the second-order nonlinear signal output from the multiplication circuit 12 into three, and two of them are input. The other one of the second-order nonlinear signals branched into three is input to the filter 13. The multiplication circuit 16 multiplies two input second-order nonlinear signals. The multiplication circuit 16 multiplies the same signal to perform a square operation to calculate a quadruple signal (fourth power signal). The multiplier circuit 16 outputs the calculated fourth-order nonlinear signal to the filter 17. The filter 17 extracts an envelope component from components included in the fourth-order nonlinear signal output from the multiplication circuit 16 and outputs the extracted envelope component to the level adjusting units 18 and 28. Similarly to the filter 13, the filter 17 removes unnecessary components and noise generated when the signals multiplied by two in the multiplication circuit 16 are multiplied.

  The level adjustment unit 18 performs level adjustment on the signal output from the filter 17. The level adjustment in the level adjustment unit 18 is the same as the level adjustment in the level adjustment unit 14. The signal output from the level adjustment unit 18 is combined with the signal output from the level adjustment unit 14 and the constant voltage output from the level adjustment unit 15. The first adjustment signal obtained by the synthesis is input to the vector modulator 32. The level adjustment unit 28 performs level adjustment on the signal output from the filter 17. The level adjustment in the level adjustment unit 28 is the same as the level adjustment in the level adjustment unit 14. The signal output from the level adjustment unit 28 is combined with the signal output from the level adjustment unit 24 and the constant voltage output from the level adjustment unit 25. The second adjustment signal obtained by the synthesis is input to the vector modulator 32. The level adjustment in the level adjustment units 18 and 28 is determined according to the characteristics of the amplifier provided in the subsequent stage of the predistortion circuit 3, particularly the characteristics of the fifth order term.

  According to the predistortion circuit 3, distortion that suppresses distortion generated from the third-order term and the fifth-order term among distortions generated in the amplifier provided in the subsequent stage can be given in advance to the analog signal. The characteristics can be improved. Further, according to the predistortion circuit 3, since the level adjustment units 18 and 28 are provided, distortion suppression based on the third-order term can be performed independently, and the time required for design or examination is reduced. can do.

  Note that, in the predistortion circuit 3, a configuration for generating a doubled signal to be input to the level adjusting units 14 and 24 may be provided independently as in the predistortion circuit 1 in the first embodiment. Similarly, a configuration for generating a quadruple signal to be input to the level adjusting units 18 and 28 may be provided independently. Further, in the predistortion circuit 3, a multiplication circuit is further provided so as to generate a 6-fold signal or an 8-fold signal so as to compensate for distortion caused by the seventh-order term and the ninth-order term. Good.

(Fourth embodiment)
FIG. 7 is a block diagram illustrating a configuration example of the relay device 50 according to the fourth embodiment. The relay device 50 uses the predistortion circuit 1 of the first embodiment. The relay device 50 is, for example, a non-regenerative relay-type relay station that amplifies and transmits a received radio signal in a radio frequency band (for example, 6 GHz band) without regenerating it into a digital signal. The relay device 50 includes a reception antenna 51, a reception unit 52, a predistortion circuit 1, a transmission unit 55, and a transmission antenna 58. The reception unit 52 includes a low noise amplifier (LNA) 53 and a down converter 54. The transmission unit 55 includes an up converter 56 and a power amplifier (PA) 57.

  The receiving antenna 51 receives a radio signal and outputs the received signal to the low noise amplifier 53. The low noise amplifier 53 amplifies the signal output from the receiving antenna 51 and outputs the amplified signal to the down converter 54. The down converter 54 performs frequency conversion on the received signal and converts it to a signal in an intermediate frequency band (for example, 130 MHz band). The down converter 54 outputs the intermediate frequency band signal obtained by the conversion to the predistortion circuit 1.

  The predistortion circuit 1 performs compensation for a signal in the intermediate frequency band to suppress distortion generated in the power amplifier 57 provided in the subsequent stage and to provide distortion that improves linear characteristics. The predistortion circuit 1 outputs a correction signal obtained by distorting the intermediate frequency band signal to the up-converter 56.

  The up-converter 56 converts the correction signal output from the predistortion circuit 1 into a radio frequency band signal. Upconverter 56 outputs a radio frequency band signal obtained by the conversion to power amplifier 57. The power amplifier 57 amplifies the signal input from the up-converter 56 and transmits it from the transmission antenna 58.

  In the relay device 50, the predistortion circuit 1 is provided, so that distortion generated in the power amplifier 57 can be suppressed, thereby improving the linear characteristic of the amplifier. Since the relay device 50 does not need to reproduce a digital signal, it is not necessary to provide a circuit related to digital signal processing, which is preferable in terms of cost and the like. In addition, the predistortion circuit 1 can easily cope with the change of the power amplifier 57 provided in the subsequent stage by controlling the level adjustment in the level adjustment units 14, 15, 24, and 25.

  The relay device 50 may include the predistortion circuit 2 according to the second embodiment or the predistortion circuit 3 according to the third embodiment, instead of the predistortion circuit 1.

(Fifth embodiment)
FIG. 8 is a block diagram illustrating a configuration example of the transmission system 60 according to the fifth embodiment. The transmission system 60 uses the predistortion circuit 1 of the first embodiment. The transmission system 60 is, for example, a system that transmits (broadcasts) a transmission signal generated in a performance place such as a studio of a broadcasting station as a signal in a radio frequency band. The transmission system 60 includes a transmission signal generation device 61 and a transmission device 62. The transmission signal generation device 61 is provided in a performance room and generates a transmission signal including information to be transmitted (video information, audio information). The transmission signal generation device 61 transmits the generated transmission signal to the transmission device 62 via a line. The transmission signal generation device 61 generates and transmits a transmission signal in an intermediate frequency band (for example, 130 MHz band) having a frequency lower than that of the radio frequency band in order to avoid signal degradation in transmission to the transmission device 62.

  The transmission device 62 includes a predistortion circuit 1, a transmission unit 55, and a transmission antenna 58. The predistortion circuit 1 performs compensation for the transmission signal received from the transmission signal generating device 61 to suppress distortion generated in the power amplifier 57 provided in the subsequent stage and to provide distortion that improves the linear characteristics. The predistortion circuit 1 outputs a correction signal obtained by distorting the intermediate frequency band signal to the up-converter 56.

  In the transmission system 60, by providing the predistortion circuit 1, it is possible to improve the linear characteristics of the amplifier by suppressing distortion generated in the power amplifier 57. The transmission system 60 may include the predistortion circuit 2 according to the second embodiment or the predistortion circuit 3 according to the third embodiment, instead of the predistortion circuit 1.

  In the predistortion circuit described in the first to fifth embodiments, control for determining an amplitude adjustment amount (amplification amount or attenuation amount) in each level adjustment unit based on the measurement result of the output signal by the measuring instrument. A part may be provided. In this case, the control unit determines the adjustment amount of each level adjustment unit so that a desired gain is obtained at the level of an arbitrary input signal. At this time, the IM3 component may be measured by a measuring instrument, and the adjustment amount of each level adjustment unit may be determined so that the IM3 component is minimized.

  According to at least one embodiment described above, the level adjustment units 14 and 24 that acquire the first and second adjustment signals based on the second-order nonlinear signal obtained by squaring the analog signal, and the analog signal as the first signal. In addition, by having the vector modulator 32 that modulates with the second adjustment signal, it is possible to give the analog signal distortion that suppresses distortion characteristics obtained by approximating distortion of the amplifier with a third-order polynomial. Since the level adjustment in the level adjustment units 14 and 24 can be performed independently with respect to the third-order term, it is possible to reduce the time required for measuring or examining the predistortion circuit.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

  1, 2, 3 ... Predistortion circuit, 11, 21 ... Attenuator, 12, 16, 22 ... Multiplication circuit, 13, 17, 23 ... Filter, 14, 15, 18, 24, 25, 28 ... Level adjustment unit, DESCRIPTION OF SYMBOLS 31 ... Delay element, 32 ... Vector modulator, 33 ... 90 degree hybrid, 34, 35 ... Mixer, 36 ... Synthesizer, 50 ... Relay device, 51 ... Reception antenna, 52 ... Reception unit, 53 ... Low noise amplifier, 54 ... Down converter, 55 ... Transmission unit, 56 ... Up converter, 57 ... Power amplifier, 58 ... Transmission antenna, 60 ... Transmission system, 61 ... Transmission signal generator, 62 ... Transmission device, IN ... Input terminal, OUT ... Output terminal

Claims (6)

A first multiplier for calculating a first square signal obtained by squaring an input analog signal;
A first level adjustment unit for obtaining a first adjustment signal from the first square signal by performing level adjustment;
A second level adjustment unit for obtaining a second adjustment signal from the first square signal by performing level adjustment;
Generating a first analog signal and a second analog signal having a phase difference of 90 degrees from the analog signal; a first multiplication result of the first adjustment signal and the first analog signal; A vector modulation unit that combines the adjustment signal of 2 and the second multiplication result of the second analog signal, and outputs an output signal obtained by the synthesis;
A predistortion circuit. A first attenuation unit for outputting a first attenuation signal obtained by attenuating the analog signal;
Further comprising
The first multiplier is
Calculating the first squared signal by squaring the first attenuated signal;
The predistortion circuit according to claim 1. A third level adjustment unit that outputs a third adjustment signal having a constant voltage;
A fourth level adjustment unit for outputting a fourth adjustment signal having a constant voltage;
Further comprising
The vector modulation unit is
Multiplying the first analog signal by a signal obtained by combining the first adjustment signal and the third adjustment signal;
Multiplying the second analog signal by a signal obtained by combining the second adjustment signal and the fourth adjustment signal;
The predistortion circuit according to any one of claims 1 and 2. A third multiplier that calculates the first square signal by squaring the first square signal;
A fifth level adjustment unit for obtaining a fifth adjustment signal from the first square signal by performing level adjustment;
A sixth level adjustment unit that obtains a sixth adjustment signal from the first square signal by performing level adjustment;
Further comprising
The vector modulation unit is
Multiplying the first analog signal by a signal obtained by combining the first adjustment signal, the third adjustment signal, and the fifth adjustment signal;
Multiplying the second analog signal by a signal obtained by combining the second adjustment signal, the fourth adjustment signal, and the sixth adjustment signal;
The predistortion circuit according to claim 3. A second attenuation unit that outputs a second attenuation signal obtained by attenuating the analog signal;
A second multiplier for calculating a second square signal by squaring the second attenuation signal;
Further comprising
The second level adjustment unit includes:
Obtaining the second adjustment signal from the second square signal by performing level adjustment;
The predistortion circuit according to any one of claims 1 to 4. The predistortion circuit according to any one of claims 1 to 5,
A transmission signal generation unit that outputs an analog signal in an intermediate frequency band including information to be transmitted to the predistortion circuit;
An up-converter that outputs a radio signal by frequency-converting the output signal output from the pre-distortion circuit to a radio frequency band;
An amplifier for amplifying the radio signal and transmitting it from an antenna;
With
The first level adjustment unit and the second level adjustment unit perform level adjustment based on characteristics of the amplifier.
Communication device.

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